1. Field of the Invention
The present invention relates to a comb-shape filter using an ultrasonic delay device.
2. Description of Technical Background
As shown in FIG. 3, a comb-shape filter using an ultrasonic delay device generally comprises an ultrasonic delay device consisting of an ultrasonic delay medium 1 and input and output electromechanical transducers 7, 8 (hereinbelow referred to simply as a transducer) attached some surfaces of the ultrasonic delay medium, and a branch line with an impedance element 2 which is connected between the input and output terminals of the ultrasonic delay device so that a part of an input signal is applied to the output terminal of the delay line through the impedance element 2 and without passing through the ultrasonic delay device.
A resistor 6 is to adjust the level of the signal passing through the branch line and a signal passing through the ultrasonic delay device. A resistor 9 is to match the output impedance of the ultrasonic delay device 1.
When an input electric signal is applied to the input terminal 10 of the comb-shape filter, it is converted into an ultrasonic wave at the input transducer 7. The ultrasonic wave is then transmitted in the ultrasonic delay medium 1 with a predetermined time delay, and is again converted into an electric signal at the output transducer 8 to become an output electric signal. On the other hand, a part of the input electric signal is applied to the output signal of the ultrasonic delay medium 1 through the impedance element 2. The part of the input electric signal is added to the output electric signal whereby an output electric signal having comb-shape filter characteristics is obtained from the output terminal 11.
Recently, improvement in video-equipments and picture quality have been needed particularly in the property of the comb-shape filter. For instance, there is the demand of a reproduced picture having a high resolution for a video tape recorder (VTR). It is therefore necessary to provide a television receiver to meet such demand. Resolution of the reproduced picture in the vertical direction depends on the number of scanning lines, and resolution in the horizontal direction largely depends on the number of horizontal stripes of a luminance signal factor. It is known that a high resolution can be obtained by increasing either or both of these numbers.
A video signal in the NTSC system is formed by a luminance signal factor 17 and a color signal factor 18 and has a band width as high as about 4 MHz as shown in the response frequency characteristic diagram of FIG. 4. Accordingly, a conventional comb-shape filter possesses frequency characteristics as shown in FIG. 5. Since the input and output transducers 7, 8 of the ultrasonic delay device indicate bandpass filter characteristics as shown in FIG. 6, the frequency characteristics of the conventional comb-shape filter is obtained by giving to the signal passing through the branch line frequency characteristics which is declined toward the right side, by using an inductance element and/or other elements as the impedance element 2.
Thus, the conventional comb-shape filter having a narrow band width could not provide a sufficient number of horizontal stripes, and accordingly, it was difficult to attain a high resolution.
On the other hand, for portable video-equipments such as color television receivers, there is a strong demand to miniaturize of the equipments. Likewise, miniaturization is requied for the comb-shape filter as a part of the portable video-equipment.